Also do make sure the HTML heading does not include any namespaces, a sample HTML header would be:
<?xml version="1.0" encoding="UTF-8"?>
<html xmlns=http://www.w3.org/1999/xhtml>
With all the rules catered, the sample index.html will look like:
<?xml version="1.0" encoding="UTF-8"?>
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<title>Index File</title>
</head>
<body>
<h1>Index File (this will be ditamap title)</h1>
<div class="content">
<ul class="book">
<li class="topicref"><a href="link1_file_path.html">link1_file</a></li>
<li class="topicref"><a href="link2_file_path.html">link2_file</a>
<ul class="book">
<li class="topicref">
<a href="link3_file_path.html">link3_file</a>
</li>
<li class="topicref">
<a href="link4_file_path.html">link4_file</a>
</li>
</ul>
</li>
</ul>
</div>
</body>
</html>
